Session: 03-03: Energy Storage Separate from CSP: Thermal, Mechanical, Thermochemical

Paper Number: 142372

142372 - Design-Point Techno-Economics of Pumped Thermal Energy Storage as Combined Heat and Power 

Abstract: 

Deep decarbonization of electricity likely requires significant deployment of variable renewable energy generators like photovoltaics and wind power. As these technologies account for an increasing share of capacity on the grid, there will be periods where there are mismatches between generation and demand. Grid integration research suggests that long duration energy storage (LDES) offering around ten or more hours of storage may be a cost-competitive solution to enable a robust grid with significant deployment of VREs. Pumped thermal energy storage (PTES) is a LDES technology that uses a thermodynamic heat pump cycle that utilizes low-cost electricity to charge hot and cold thermal energy stores. When electricity generation is required, PTES utilizes a power cycle that converts the stored thermal energy into electricity. PTES technologies that utilize proven power generation components may be able to achieve round-trip efficiencies (RTE) of around 50-60%. While this RTE is lower than conventional Lithium-Ion batteries can achieve, PTES offers the potential for low-cost scaling by adding hours of TES at relatively low cost.

Process heat is also an important area for decarbonization, as it accounts for around 8% of primary energy consumption in the United States. Over 50% of process heat is consumed at temperatures colder than 300°C. One unexplored option to provide heat in this temperature range is to leverage the unique capability of PTES among LDES technologies to discharge its round-trip losses as heat at useful temperatures. An idea-gas Brayton PTES system using molten salt thermal energy storage (TES) rejects heat from around 110°C to slightly above the ambient temperature. However, similar to conventional combined heat and power (CHP), small modifications to the system can increase the temperature of heat rejection while sacrificing a small amount of RTE. In this study, we evaluate two different PTES-CHP methods to increase the heat rejection temperature of the ideal-gas Brayton PTES. First, we maintain the nominal configuration and simply increase the temperature at which the cycle must reject heat. This modification can achieve heat rejection temperatures up to around 160°C. Second, we add a heat rejection immediately downstream of the turbine. This modification can achieve heat rejection temperatures above 300°C, although some of the total PTES heat rejection must occur at a second, lower-temperature heat exchanger to balance the cycle.

When we attribute the extra electricity consumption caused by the reduction of RTE exclusively to the heat generation, we find that the resulting conversion of electricity to heat surpasses Carnot and Lorenz coefficients of performance. This outcome results from the fact that the nominal PTES design generates a significant amount of heat with no electricity penalty and our PTES-CHP design boosts the temperature of the entire heat output at a relatively low level of electricity consumption. Accordingly, we find that when accounting for the cost of electricity purchases, a heat rejection heat exchanger, and TES between the cycle and heat off-taker, the PTES-CHP concept can achieve competitive levelized costs of heat up to 300°C.

Presenting Author: Ty Neises NREL

Presenting Author Biography: Ty is a senior researcher at NREL focused on thermal energy systems.

Authors:

Ty Neises NREL
Joshua Mctigue NREL